The rule of cool, if something is cool then you are totally free to add it to your setting. And lots of players love to "wear the exclusive dress".
And we can't forget the PC races from Dragon Magazine when this was published by Paizo.
Today WotC's team worries more about a perfect design of new PC races to become popular among the players. Not only right racial traits, avoiding the typecasting but also to be an interesting concept. Hasbro wants the new PC races also to sell toys and other merchandising products.
If now kobolds can be PCs or no-so-evil, for example as metalic dragons' minions, what about the animosity between kobolds and gnomes, by fault of the past conflict between their deities?
An Eberron-Spelljammer official crossover wouldn't be totally imposible, and I say it seriously. They share some common elements.
I imagine the lore of the bugbears like the gorillas from "the planet of the apes", and the goblins like the "losers" who could survive a zombie apocalypse (at least because thanks their inmunological system they need a lot of time more to be totally infected and sometimes even to survive the bite).
If the PC races are going to lose racial atributes modifiers, why not ones based in the "life style" (living in the wilds, academy, underground, cold weather...)?
Changelings and sifhters totally possible in Ravenoft, and even the kalashtars could be humans who survived slavery in Bluetspur domain.
Nothing of Dark Sun without previous UA psionic powers, and also we will have to see an UA about psionic PC races (dromites, maenas, xephes, elans, and even maye shardminds, fraals or blues! (goblin subrace). (elans as PC in Dark Sun may be too broken). Reborns in Kalidnay(dark domain)?
No Planescape without a previous UA about PC races as rogue modrons, bariaurs, spikers, mephlings, para-genasies, mechanatrixes and other planetouched) (but the petitioners' offspring "wildren" may need a reboot/retcon).
* Shouldn't centauresses ( = female centaurs) to wear skirts to cover that?